The present invention relates to a method for determining the minority carrier surface recombination lifetime constant (t.sub.s) of a specimen of semiconductor material.
Metals such as ion and copper are common problems in manufacturing silicon devices. Small quantities of these element which find their way into product wafers during processing cause lowered voltage breakdown values and reduced device yield. The value of the minority carrier surface recombination lifetime constant (t.sub.s) is well known to be sensitive to semiconductor metallic contamination. The (t.sub.s) value is also known to be sensitive to damage of crystalline order of the silicon-silicon oxide interface boundary.
A number of methods are known for measuring (t.sub.s). These include photoconductivity decay (PCD), CV pulse, surface photovoltage (SPV) using various wavelengths of light, short-circuit current open-circuit voltage decay, photoluminescence decay and election beam induce current decay.
In U.S. Pat. No. 4,827,212 to E. Kamieniecki, which patent is incorporated herein by reference, there is described a method and apparatus for characterizing a semiconductor using the surface photovoltage (SPV) effect. A region of the surface of a semiconductor is illuminated with an intensity modulated beam of light, the wavelength of the light being shorter than that corresponding to the energy gap of the semiconductor. The surface photovoltage (SPV) induced in the semiconductor is measured under varying bias voltage conditions. The intensity of the light beam and the frequency of modulation are selected such that the surface photovoltage (SPV) is directly proportional to the intensity and reciprocally proportional to the frequency of modulation. The apparatus includes a pair of electrodes, a light source, a modulated power supply, a dc bias voltage source, an amplifier, a phase sensitive detector, an A/D converter and a computer. Using the surface photovoltage (SPV) and the bias voltage (V.sub.g) measurements, the charge induced in the semiconductor space charge region (Q.sub.se) and the charge induced in the semiconductor (Q.sub.ind) are determined. This information is used to determine certain specified parameters of the semiconductor such as surface state density and oxide/insulator charge doping type and doping concentration. The technique is designed especially for use in characterizing semiconductor wafers, coated or uncoated, but may, if desired, also be used in characterizing MIS or MOS type semiconductor devices.
In U.S. Pat. No. 4,891,584 to E. Kamieniecki et al there is described an apparatus for making ac surface photovoltage (SPV) measurements of a specimen of semiconductor material under dc bias voltage conditions which includes a light source whose output beam is intensity modulated, an adjustable dc bias voltage source, a conductive base for supporting the specimen and a novel capacitance type reference electrode assembly for sensing the SPV signals. The reference electrode assembly includes in one embodiment a transparent flexible sheet of insulating material having on one surface a first conductive coating which serves as a reference electrode and a second conductive coating which serves as a guard electrode, the first coating being transparent. The flexible sheet of insulating material is attached to a flat glass plate through an annular spacer. An elastomeric button is positioned between the sheet and the glass plate. When the SPV measurements are being taken the flexible sheet is pressed against the specimen with pressure sufficient to hold the reference electrode in close compliance with the specimen, with the pressure being applied to the glass plate and being transmitted from the glass plate to the flexible sheet of insulating material through the elastomeric button.
In U.S. Pat. No. 4,333,051 to A. Goodman there is disclosed a method and apparatus for determining the diffusion length of minority carriers in semiconductors using the constant-magnitude surface-photovoltage (SPV) method. A servo system maintains a constant predetermined value of the SPV. A novel capacitance-pickup electrode couples the SPV to a preamplifier in the measurement system and functions to reduce SPV drift. A keyboard or computer is used to select both the operating optical wavelength of an adjustable monochromator and a network to compensate for the wavelength dependent sensitivity of a photodetector used to measure the illumination intensity (photon flux) on the semiconductor. Measurements of the relative photon flux for a plurality of wavelengths are plotted against the reciprocal of the optical absorption coefficient of the material. A linear plot of the data points is extrapolated to zero intensity. The negative intercept value on the reciprocal optical absorption coefficient axis of the extrapolated linear plot is the diffusion length of the minority carriers.
In U.S. Pat. No. 4,649,339 to R. Gangrath there is disclosed an interface for testing an integrated circuit device through a plurality of contacts on an exposed surface of the device. The interface comprises a flexible sheeting having first and second opposing surfaces. A plurality of contacts are disposed on the first surface of the sheet in a pattern designed to mate with a plurality of contacts on an integrated circuit device. A plurality of thin film conductors are patterned on the sheet, the conductors each being connected to a contact on the sheet and being adapted for connection to a circuit for testing the device. The interface comprises apparatus for preforming into a generally domed shape a portion of the sheet which includes a plurality of the contacts and apparatus for flexing the flexible sheet so that the contacts on both the sheet and the device may be brought into intimate contact.
In U.S. Pat. No. 5,091,691 to E. Kamieniecki et al there is disclosed an apparatus for making ac surface photovoltage (SPV) measurements of a specimen of semiconductor material under dc bias voltage conditions. This apparatus includes a light source whose output beam is intensity modulated, an adjustable dc bias voltage source, a conductive base for supporting the specimen and a novel capacitance type reference electrode assembly for sensing the SPV signals. The reference electrode assembly includes in one embodiment a bottom made of insulating elastomeric material and attached to a rigid plate made of insulating material. A film made of insulating material and having a conductive coating on one side which serves as a reference electrode is attached to the button. When SPV measurements are being taken, the film is pressed against the specimen with pressure sufficient to hold the reference electrode in close compliance with the specimen, with pressure being applied to the plate from an external source and being transmitted from the rigid plate to the film through the elastomeric button.
In U.S. Pat. No. 4,393,348 to B. Goldstein et al there is disclosed a method and apparatus for determining the diffusion length of minority carriers in semiconductor material, particularly amorphous silicon which has a significantly small minority carrier diffusion length, using the constant-magnitude surface-photovoltage (SPV) method. An unmodulated illumination provides the light excitation on the surface of the material to generate the SPV. A vibrating Kelvin method-type probe electrode couples the SPV to a measurement system. The operating optical wavelength of an adjustable monochromator to compensate for the wavelength dependent sensitivity of a photodetector is selected to measure the illumination intensity (photon flux) on the silicon. Measurements of the relative photon flux for a plurality of wavelengths are plotted against the reciprocal of the optical absorption coefficient of the material. A linear plot of the data points is extrapolated to zero intensity. The negative intercept value on the reciprocal optical coefficient axis of the extrapolated linear plot is the diffusion length of the minority carriers.
In U.S. Pat. No. 5,087,876 to L. Reiss et al, which patent is incorporated herein by reference, there is disclosed an apparatus for making ac surface photovoltage (SPV) measurements of a specimen of semiconductor material under variable dc bias voltage conditions. The apparatus includes a light source whose output beam is intensity modulated, a reference electrode, a guard electrode, a back electrode, a first voltage for biasing the reference electrode with a variable dc voltage and a second voltage separate from the first voltage for biasing the guard electrode such that an accumulation layer is established in the area on the specimen controlled by the guard electrode to prevent flow of carriers between the area controlled by the reference electrode and the rest of the specimen.
It is an object of this invention to provide a new and improved method for determining the minority carrier surface recombination lifetime constant (t.sub.s) of a specimen of semiconductor material.
It is another object of this invention to provide a method as described above which is fast and reliable.
It is still another object of this invention to provide a method as described above which can be practiced using apparatus similar to that described in the above noted Kamieniecki and Reiss patents.